CVS is performed at 10 to 14 weeks of pregnancy by taking a small sample of placental tissue (chorionic villi). Cells found in the chorionic villi almost always have the same chromosomes as the baby. Therefore abnormalities found in the chromosomes of the chorionic villi are considered to occur in the baby.
The procedure is performed under local anaesthetic which numbs the area of skin prior to inserting the needle. The needle is inserted through the skin of the abdomen and is observed at all times by ultrasound. The needle does not enter the amniotic sac or go near the baby.
In some situations a CVS is performed through the cervix. Local anaesthetic is not required as the instrument passes through the cervical canal. A speculum is used, similar to having a pap smear, so that the cervix can be visualised. The instrument is observed at all times by ultrasound.
Risks associated with CVS
- The risk of pregnancy loss due to a transabdominal CVS is between 0.5 and one per cent (between one in 100 and one in 200 procedures).
- The risk of pregnancy loss due to a transcervical CVS< is up to two per cent (one in 50 procedures).
- Light bleeding can occur after a CVS but usually settles without further problems.
- In one per cent of cases, a CVS result may be difficult to interpret due to a situation called mosaicism. This uncertainty can be resolved by performing an amniocentesis.
- In rare cases a result can not be provided from a CVS and repeat sampling may be required.
Amniocentesis is performed after 15 weeks of pregnancy by taking a sample of amniotic fluid that surrounds the baby. Amniotic fluid contains cells from the baby’s skin.
The amniocentesis procedure
Amniocentesis is performed by inserting a needle through the skin of the abdomen. The needle is observed at all times by ultrasound. Approximately 20 ml of amniotic fluid is collected.
Risks associated with amniocentesis
- The risk of pregnancy loss due amniocentesis is between 0.5 and one per cent (between one in 100 and one in 200 procedures).
- Infection following amniocentesis is very rare and occurs in less than one in 1000 procedures performed.
- In rare cases a result can not be provided from amniocentesis and repeat sampling may be required.